The present invention relates to copying machine, and more particularly, to an improvement of a projection device employed therein for projecting light emitted from a light source onto a document to be copied.
In the field of photocopying, it is considered convenient to provide a dual-purpose photocopying machine which may produce photocopies both of thin original documents, for example in the form of a single sheet of paper, and of thick original documents, for example in the form of a page of a book, in order to avoid the expense and space requirements of separate photocopying machines exclusive for thin or thick original documents. When a thick original document, for example, a page of a book is to be copied, the reverse side of the document is protected from external light by other pages of the book, etc., with which the document being copied is combined, while the document is held in position for photocopying by the weight of such other pages, etc. On the other hand, when a thin original document is copied, it is necessary to employ means for positioning and holding the original document in the required manner and also to protect the original document from external light, in order to avoid excessive illumination of the document during photocopying thereof.
In a photocopying machine in which an original document is moved during the photocopying process, there is employed, for example, a rotatable drum having a peripheral layer which is made of photosensitive material, and successive portions of which are exposed to image-wise light reflected from successive portions of the original document brought to an illumination station where the document is successively illuminated. The light rays reflected from the document are directed onto the photosensitive drum through a focusing lens or similar means, as the drum rotates and document is moved to form an electrostatic latent image of the document on the photosensitive peripheral layer of the drum. After the image of the original document has been transferred onto a transfer material, for example, a copy paper sheet, the copy paper is processed in a known manner and forwarded to be discharged out of the photocopying machine. In dual-purpose photocopying machine employing such means, during photocopying of a thin original document, the document is moved past the illumination station slidingly over a transparent support, while gripped between driving rolls and pressing rolls, in which the pressing rolls are provided on the inner side of a movable opaque cover which is normally positioned over the illumination station. When a thick original document is to be copied, the opaque cover is removed, and the original document is supported on a transparent carrier plate which rides on and is moved by the driving rolls to bring successive portions of the original document into alignment with the illumination station.
In the electrophotocopying machine as described above, an optical system for projecting the image of the original document onto the photosensitive layer is normally so arranged as to have the image of the document placed directly over the transparent support sharply focus on the photosensitive surface. Accordingly, the document presented in a form of sheet of paper and placed directly on the transparent support can be sharply focused on the photosensitive surface, whereas the image of the document presented in a form of book and placed on the transparent carrier plate will be out of focus due to difference of length of the optical path, which difference is caused by the thickness of the transparent carrier plate. More specifically, when the refractive index of the material of the transparent carrier plate is designated as n.sub.1, and the thickness thereof as d.sub.1 , the length of the optical path from the surface of the document in the form of book to the focusing lens or similar means is d.sub.1 /n.sub.1 longer than the length of the optical path from the surface of the document in the form of sheet of paper to the focusing lens.
In the conventional type of electrophotographic copying machines, the arrangement as described above results in a disadvantage that the focusing means of the projection means can only be set to obtain optimum sharpness of images in copying either thin original documents or thick original documents, but not both and, therefore, it has not been possible to obtain equal resolution in copying of all types of documents.
In order to overcome such disadvantages, there have been proposed various methods such as employment of a lens assembly having a long focal length so as to increase the depth of field through which focus may be achieved. However, it has been found that the employment of such a lens assembly does not ensure satisfactory resolution in photocopies of all types of documents. In addition, such employment of such a lens assembly resulted in another disadvantage that the optical system must be made larger if a lens having a long focal length is included therein.
Another method which is disclosed, for example, in U.S. Pat. No. 3,510,218 of W. Limberger et al. is to provide another transporting path for transporting the thin original documents in addition to the transporting path for transporting the thick original document, the former being arranged at a position higher than the latter by a length corresponding to the thickness of the transparent carrier plate, or a further method which is disclosed in U.S. Pat. No. 3,741,646 of Knechtel et al. is to provide a transparent carrier plate for copying thin original documents similar to the transparent carrier plate provided for copying thick original documents, so that in either method, the focal length of the optical path in relation to the thin original documents and thick original documents is the same, while such arrangements as described above often require a large space for employing additional members and result in high manufacturing cost.
A still further method is disclosed in U.S. Patent Application Ser. No. 740,722 of Ogawa et al. assigned to the same applicant, in which it is explained that the difference of the optical path length caused by the thickness of the transparent carrier plate is eliminated by placing a transparent block having a predetermined refraction index between the illumination station and focusing means.
However, in addition to such disadvantages as previously explained, the presentation of the transparent carrier plate results in a decrease of light intensity of the light rays reflected from the document presented in the form of book since a certain portion of light rays are absorbed or reflected by the transparent carrier plate, thus resulting in a difference in the condition of copied papers between those copied with and without the transparent carrier plate.
It is accordingly, a primary object of the present invention to provide a projection device for use in electrophotographic copying machine which is designed to directly transmit a thin original document and a thick original document, where the thick original document is placed over a carrier member, said projection device including correction means for correcting the changes of the light of the optical path and the amount of exposure on the photoreceptor caused by the presence of the carrier member.
It is another object of the present invention to provide a projection device of the above described type which is provided with a ventilation system for cooling the various elements provided in the projection device so as to avoid an undesirable heating of the projection device caused by the employment of the correction means.
It is a further object of the present invention to provide a projection device of the above described type which is compact in size accurate in functioning and can be produced at low manufacturing cost.